This invention relates generally to electrical circuitry and in particular to a circuit and a method for controlling the speed of an alternating current motor.
In the past, various types of refrigeration systems and controls therefor have been utilized for cooling a given space. In general, the past refrigeration systems were provided with an expansion valve for the expansion of a liquid refrigerant supplied thereto from a condenser into a gaseous state which was then delivered through an evaporator for cooling purposes to a compressor. The compressor pumped the system refrigerant through a plurality of coils in the condenser where it was condensed back to its liquid state. A fan motor was also utilized in the refrigeration system to flow ambient air generally in a path over the coils of the condenser to aid in the condensation of the generally gaseous refrigerant flowing through the condenser coils from the compressor toward the expansion valve. It is well known that the efficiency of a refrigeration system may be impaired thereby to create other problems if the condenser temperature varies substantially from a preferred or selected temperature value or a temperature value range.
It is desirable to pass enough air over the condenser coils to maintain the refrigerant in a gaseous state generally at the compressor end portion of the condenser coils and in a liquid state generally at the expansion valve end portion of the condenser coil for supplying the liquid refrigerant to the expansion valve. Of course, the amount of air flowed by the fan motor over the condenser coils to maintain this balance between the gaseous and liquid phases of the refrigerant in the compressor coils is a function of the temperature of the ambient air flowed by the fan motor over the condenser. By properly controlling the speed of the fan motor, the head pressure of the compressor may be maintained generally constant thereby to provide proper refrigerant pressure at the inlet side, i.e. the condenser side, of the expansion valve. One such related circuit for controlling the speed of a fan motor is disclosed in my aforementioned copending application Ser. No. 548,765 filed Feb. 10, 1975, now U.S. Pat. No. 3,867,192.
In some of the past refrigeration systems, the condenser coil was tapped or communicated with a pressure responsive device which was operably connected with means for varying the speed of the system fan motor. At least one of the disadvantageous or undesirable features of this past type of system and motor fan speed controlling scheme was the creation of another joint or tie into the system which increased the possibility of refrigerant leakage therefrom. Another disadvantageous or undesirable feature of this past type of system and fan motor speed controlling scheme was that another mechanically operating mechanism was included thereby to increase the likelihood of system failure due to mechanical malfunctions.
In other past refrigeration systems, the temperature of the condenser coil was utilized for controlling fan motor speed since such temperature may be correlated with the pressure of the refrigerant in the system in an attempt thereby to maintain the head pressure of the compressor generally constant. In many of this past type refrigeration systems and fan motor speed controlling schemes, the condenser coil temperature was measured at a selected location generally defining the interface of the gaseous and liquid phases of the refrigerant in the condenser coils by means, such as a thermistor or the like, attached by various suitable means to the condensor coil sensing the temperature at the phase interface. At least one of the disadvantageous or undesirable features of this particular past type of refrigeration system and fan motor speed control scheme is believed to be that the interface of the gaseous and liquid phases of the refrigerant varied location-wise in the condenser coil wherein the temperature sensing means in its selected phase interface location on the coil was subjected to temperatures of the refrigerant other than that selected at the phase interface thereof.